1. Field of the Invention
The present invention relates to a color image forming apparatus, and in particular, to a color image forming apparatus in which electrostatic latent images are formed on photosensitive bodies by light beams being scanned by being deflected in a predetermined direction by a plurality of light scanning means disposed in correspondence with the plurality of photosensitive bodies which are provided in correspondence with colors to be developed, and the electrostatic latent images formed on the respective photosensitive bodies are developed by coloring materials of colors corresponding to the photosensitive bodies, and the developed images of the respective colors are superposed together and transferred onto a transfer medium such that a color image is formed.
2. Description of the Related Art
Conventionally, a color image forming apparatus is provided with a plurality of photosensitive bodies (e.g., four photosensitive bodies which are Y (yellow), M (magenta), C (cyan), and K (black)). Electrostatic latent images are formed on the photosensitive bodies of the respective colors by laser beams being turned on and off on the basis of image information of the respective colors by light scanning devices corresponding to the respective photosensitive bodies. By successively superposing toner images developed by developing means onto a single transfer medium, a desired color image is formed.
At this time, if the images which are superposed onto the single transfer medium are offset per color (referred to hereinafter as "color offset"), the hue changes, color unevenness occurs, and the like, such that the desired output image cannot be obtained. Reasons for color offset include offset of the light scanning positions due to the temperatures or the like of the light scanning devices, skewing or bowing of the scan lines of the respective light scanning devices, offset of the positions at which writing of the respective colors begin, offset of the light scanning positions due to uneven speeds of the photosensitive bodies or the conveying bodies, vibration or the like, changes over time, or the like. Among these, factors which most markedly affect image quality are skewing and bowing of the scan lines of the light scanning devices and offset of the positions at which writing of the respective colors begin.
Conventionally, in order to correct such offset, a patch image for detecting offset amounts of scan lines is formed on the transfer medium at the time the power source is turned on or at the time of the print cycle. By reading the patch image by a CCD, the relative amounts of offset from a reference color can be detected. Two CCDs are disposed at both ends of the image forming region on the transfer medium. By detecting offset at two points, the skewing of the scan lines of the respective light scanning devices can be detected.
In Japanese Patent Application Laid-Open No. 1-183676, subscanning direction offset from a reference color is corrected by moving, parallel and in the respective subscanning directions, bend-back mirrors of the light scanning devices other than the reference color, by using stepping motors and in accordance with the relative amounts of offset from the reference color. Further, there is a technique for correcting skewing by moving the stepping motors, which are at two places at the left and the right of the bend-back mirror holding portions, in accordance with the detected amounts read at CCDs (the amounts of offset). Further, in Japanese Patent Application Laid-Open No. 2-96780, from the detected results read by CCDs and in accordance with the amounts of skewing from the reference color, correction is carried out to make the color offset inconspicuous by tilting f.theta. lenses at the light scanning devices of the other colors to the left and the right by stepping motors so as to make the amounts of skewing coincide with the reference color. In addition, there are structures in which, other than stepping motors, piezoelectric elements are used in order to correct the color offset by tilting and moving the mirrors. In this way, it has conventionally been common to correct color offset by mechanical correction.
Bowing of scan lines of a light scanning device occurs due to the optical layout and the planarity of the mirror, the deflection of the mirror, and the like. Conventionally, the optical parts have been designed such that bowing of the scan lines is corrected.
Further, it is conventionally common to effect adjustment by one pixel unit in the subscanning direction by adjusting a predetermined signal count value (for starting image writing). In addition, in Japanese Patent Application Laid-Open No. 6-246975, adjustment of less than or equal to one pixel unit is carried out by controlling the rotation phases of rotating polygon bodies (hereinafter called "scanner motors") which deflect the laser beams of the light scanning devices of the respective colors.
However, in the inventions disclosed in Japanese Patent Application Laid-Open No. 1-183676 and Japanese Patent Application Laid-Open No. 2-96780 and the like, correction is carried out so as to eliminate color offset by mechanical control. In this method, the correction cycle requires an extremely great amount of time. As a result, there is the problem that it cannot be executed during a regular print cycle. In actuality, the correction cycle requires several minutes. If this correction cycle is executed during a print cycle, the user is made to wait during this time. In particular, in cases in which minute correction is carried out for each color, even more time is required.
Accordingly, the mechanical correction as described above is executed only at the time of initial setting when the power source of the image forming device is turned on. Namely, there is the problem that the amount of offset which is caused by fluctuations in the positions of the scanning beams caused by temperature or the like during the print cycle, or by variations over time, or the like cannot be corrected.
Further, as one more reason why the correction cycle cannot be executed during the print cycle, there is the following reason. Namely, in the above-described correction method, for example, when skewing of the scan lines of the light scanning devices of the respective colors is corrected, registration (reference position) in the subscanning direction is offset. In this way, there is the problem that plural parameters cannot be controlled independently.
Namely, when correction is carried out mechanically as in the conventional art, too much time is required to carry out minute correction, and if attempts are made to complete the correction cycle in a short time, only rough correction can be carried out. Namely, with mechanical control, there is a trade-off between precision and speed. When aiming for high image quality and high image speed, this is fatal, and meeting of the user's requirements cannot be achieved. Further, there is also the problem that the device becomes large-scale.